That should work, but if you want a unique array then hashes will be the simplest way to go e.g 
my @ids     = qw/ A1 A2 A3 B5 B6 B7 C789 C790 /;
my @letters = sort keys %{ { map { substr($_,0,1), undef } @ids } };

print "Found the following letters: @letters\n";

__output__

Found the following letters: A B C

[download]
See. Perl Idioms Explained - keys %{{map{$_=>1}@list}} for an explanation of the above technique for simply getting a unique list.
HTH

_________
broquaint

update: fixed bug as noted below by davido

In reply to Re: slurping into a substring'ed' array by broquaint
in thread slurping into a substring'ed' array by seaver

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